The burning of fuel to produce energy, and particularly mechanical energy, is at the root of modern society. Improvement in the efficiency of such combustion, or in reduction of the emissions created by combustion, are therefore important. A variety of prime movers or engine types are currently in use. The most widespread of these are the internal combustion engine and the turbine.
The internal combustion engine, especially the spark-fired “Otto cycle” engine, is particularly ubiquitous, but presents significant challenges in the further improvement of its efficiency. The reciprocating piston Otto cycle engine is in principle extremely efficient. For example, an Otto cycle engine operating with a 10:1 compression ratio, constant volume TDC, no heat loss, and at constant specific heat ratio (K) should, in theory, have about a 60% cycle efficiency. However in actual practice, engines typically operate at about half these air cycle values (i.e. about 31–32% efficiency). This is due to a number of reasons, including the fact that as the fuel burns, raising air temperature, the combustion chemistry limits peak temperature through dissociation and specific heat increase. Also, heat loss, finite burning, and exhaust time requirements reduce efficiency to about 85% theoretical fuel-air cycle values. Finally, engine friction, parasitic losses, etc., reduce actual power output by another 15% or so in a naturally aspirated engine.
It is well-known that it would be more efficient to run such an engine leaner—i.e., at a higher stiochiometric ratio of oxygen to fuel—to improve efficiency and reduce NOx (nitrogen oxide) emission. However, lean burning makes it difficult to sustain flame-speed (and thus avoid misfire) in a conventional Otto cycle engine, which limits the effectiveness of this approach. This problem could be overcome to some extent by “supercharging” the engine—i.e. running it at an inlet pressure significantly above atmospheric pressure—but then the problem of premature detonation must be avoided, which limits the maximum available compression ratio, and thereby decreases the efficiency.
Moreover, each improvement in compression and leanness tends to increase the creation of NOx at a given peak temperature, which must then be removed by parasitic devices, such as exhaust emission systems. Further, the exhaust emission catalysts tend to be made inefficient, or poisoned entirely, by excess oxygen.